Update the AWS auth backend acceptance tests to account for the new
`iam_tags` field that comes back on responses.

* marked only tests requiring creds as acceptance

Renamed TestBackend_* to TestAcceptanceBackend_* if the test requires
AWS credentials. Otherwise left the name as TestBackend_* and set
`AcceptanceTest: false`.

* ensure generated names aren't too long

IAM roles and users have a 64 character limit, and adding Acceptance
to the test names was putting some over the length limit, so modified
generateUniqueName() to take a max length parameter and added
functions for each type of name generation (user, role, group).